Through the last 35-40 years bone cement has been used for anchoring prosthesis components when inserting artificial hip joints. Good results have been achieved with this technique, but after long time observation of large groups of patients, problems have been detected with loosening of the inserted prosthesis component. Furthermore, a considerable bone loss has occurred around the prosthesis component so that a replacement surgery is made difficult.
Through the years, alternative techniques in the shape of uncemented sockets, where the socket is anchored in acetabulum, have been developed. Different anchoring methods have been developed, and the method appearing to provide good results on short as well as long view is the use of metal sockets, preferably made of materials as e.g. titanium or vitalium. These metal sockets are typically hemispherical with a porous surface.
The surface of the metal socket is applied a so-called porous coating implying that the surface is rough and provides possibility of ingrowth of bone cells into the surface of the metal socket. The surface coating of a metal socket typically has a pore size of 50-250 μm and may either consist of small metal balls or metal fibres that are compressed or sprayed on. A supplementing hydroxapatite coating promotes the bone ingrowth process.
When inserting an uncemented porously coated metal socket, a hole fitting the metal socket to be inserted is milled in acetabulum. E.g. if acetabulum is milled up to 60 mm, a 60 mm metal socket is inserted. Then the metal socket is anchored further with 2-4 screws through holes in the metal socket. However, screw anchoring has appeared to have unfavourable effects, as the polyethylene decomposition product can migrate through the screw holes and out into the bone, and here produce osteolysis or necrosis of the bone.
In order to avoid the latter, in later years another anchoring method has been used, consisting in that a so-called “press-fit” technique has been used, where e.g. acetabulum is milled up to 60 mm in diameter, where subsequently is inserted e.g. a 62 mm or 64 mm metal socket. A great tension is thereby achieved between bone and metal socket, whereby the metal socket is held in place without supplementing use of screws. In the course of the first weeks after the operation, the bone then grows from the pelvis into surface coating of the metal socket and ensures the so-called secondary anchoring.
The described technique is, however, rather demanding and requires an experienced physician to perform the hip prosthesis surgery, as there is risk of problems if the metal socket has not been correctly inserted. If the metal socket is not securely clamped, there is risk that it may loosen in the course of the first days after the operation. If the metal socket is too strongly clamped, there is great risk of fracture in the pelvis bone around the metal socket whereby it becomes unstable.
Therefore, it is important to insert the right size of metal socket. In order to determine which size of metal socket to be used, previously a template or a test prosthesis has been used and placed in acetabulum and which by a light pressure is fixed in acetabulum, after which is selected a metal socket in the same size or with 1-2 mm oversize.
However, it is not a good and objective method of measuring, and one may risk to use/insert more than one metal socket in order to let it fit optimally. A great disadvantage by this method is that a metal socket that has been in contact with a patient cannot be autoclaved and used for another patient. This metal socket is to be discarded, which also implies increased costs in connection with a hip prosthesis surgery.
In patent publications FR 2 684 287 and U.S. Pat. No. 5,141,512 there is described equipment for measuring in connection with placing of a hip socket, but not decidedly measuring the size and the elasticity of acetabulum.
Until now, no equipment has been developed with which the physician can measure the hip socket and thereby objectivise this part of the procedure of hip surgery. E.g. the risk of fracture in the hip socket depends on the elasticity in the bone and the size of the acetabulum. Furthermore, it also has great significance whether it is a first time operation or a replacement operation.